1. Field of the Invention
This invention relates to apparatus for establishing a uniform, predetermined charge on successive image segments of a charge-receiving element.
2. Description of the Prior Art
Although the corona-charging apparatus of the present invention has general applications, one preferred application is in the field of electrophotographic apparatus (herein called copiers). In copier corona-charging apparatus, current from a corona-emitting electrode establishes a generally uniform electrostatic charge on an image segment of a charge-receiving element having a photoconductive insulating layer. The charged segment is then advanced to an exposure station where it is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and substantially transferred to paper upon which the copied image is to appear.
Various methods are known for adjusting the charge on a charge-receiving element by the corona-charging apparatus. For example, the speed of the charge-receiving element past the corona-emitting electrode can be varied to adjust its time of exposure to corona current. Another method for adjusting the charge is to vary the charging rate. Control electrodes (known as grids), to which has been applied a potential approximately equal to that to which the charge-receiving element is to be charged are located between the corona-emitting electrode and the element. Current flow from the electrode to the element can be adjusted by varying the voltage of the grid.
Very possibly, the optimum grid voltage changes between successive original documents, and high-quality reproduction can best be obtained when the charging rate is adjusted between image segments to compensate for variances in background density of the original documents. Such adjustment of the charging rate between image segments is not new, and has been effected by changing the grid voltage after the trailing edge of one image segment has cleared the corona-charging apparatus and before the leading edge of the next image segment reaches the apparatus. However, this process undesirably requires the charge-receiving element to contain a physical transition region between the trailing edge of one image area segment and the leading edge of the next image area segment so that no portion of either image area segment is below the corona-charging apparatus during the period of grid voltage change. Thus, the image area segments on the charge-receiving element must be spaced apart by a distance equal to at least the width of the corona-charging zone (i.e., the dimension of the charge-receiving apparatus in the direction of movement of the charge-receiving element).
The need for providing such spaces between the successive image area segments reduces the number of image area segments which can be located within a predetermined length of charge-receiving element. This in turn necessitates driving the charge-receiving element at a faster linear speed to make a predetermined number of copies per unit time than would be necessary if the image area segments were closer together in order to produce the same number of copies per unit time.